Cleanup Plugin Manager dialog

Merge "Name" and "Description" columns, and move plugin details to
tooltips.
Close documents in sidebar with middle mouse button
Merge branch 'scriptum/sidebar-middle-button-close'
Disable Markdown support in our Doxyfile

At least the "Plugin Writing Guidelines" and "Plugin HowTo" sections
are displaying undesirable side effects of Markdown support, so disable
it at least for now.
Strip double leading spaces

These are weird and break Doxygen Markdown support.
Strip more double leading spaces
Support out-of-tree builds by not hardcoding paths
Improve Doxygen support in Autotools build system

* Add `m4/geany-doxygen.m4` file
* Support `DOXYGEN` environment variable setting which Doxygen to use
* Support `--enable-api-docs` to activate building Doxygen docs
    (off by default)
Use improved Doxygen Autotools support
Remove some redundant path info from `doc/Makefile.am`
Re-enable Markdown support now that major bugs are fixed

I went through each link from the API docs and everything seems fine
since the extra leading whitespaces were removed in commits
`edaa6f713f1b8e037a56652da15032aac56e2227` and
`67ae7707364ecc09da8675803b48575d61b82fcf`.
Remove comment noise from `doc/plugins.dox` and make it a build depend
Add foreach_document(), documents[] examples
Fix sign comparison warning
Fix unsigned >= 0 comparison warning
Fail if Doxygen is enabled but not found
Merge branch 'ntrel/doc-macro-examples'
Fix doc/Doxyfile.stamp rule portability

$< is only portable in suffix rules, so don't use it there.
Remove unnecessary $(builddir) prefixes
Force enabling API docs on distcheck